(1) Field of the Invention
The present invention relates to an optical sound recording system of modified variable area type used for recording sound signals on a cinema film and others for which it is required strictly to reduce distortion of recorded sound signals.
(2) Description of the Prior Art
In conventional optical sound recording systems, as shown, for instance, in the Focal Encyclopedia of the Film and Television Techniques: Focal Press, London and New York, 1969, pp. 700- 703, a triangular light image formed by throwing a light of an incandescent lamp on a triangular window provided on a mask is reflected by a mirror of a galvanometer and then focused on a slit provided on another mask, so that rectilineal light images having various lengths corresponding to rotation angles of the mirror of the galvanometer which is driven by sound signals can be derived from the slit. Consequently, rectilineal light images having various length corresponding to sound signals can be projected on a film transported continuously, so as to perform sound recording of variable area type.
However, in the optical sound recording system mentioned above, the galvanometer provided with the considerably weighty mirror rotating in response to applied signals is used for modulating the light projected on the film, so that it is difficult to obtain an optical pattern recorded on the film exactly in response to sound signals of sufficiently wide frequency range.
Furthermore, in the optical sound recording system mentioned above, it is difficult to obtain the triangular light image having sufficient intensity and uniformity of brightness, even if a laser beam source is used as a light source, so that the brightness of rectilineal light images is varied in accordance with their length, and consequently the conventional optical sound recording of variable area type mentioned above is fairly inferior with respect to quality of recorded sound signals in comparison with that of a magnetic sound recording system.
By the way, in a conventional optical sound recording system of variable density type is further inferior with respect to noise and distortion of recorded sound signals in comparison with those of variable area type.
Accordingly, in case the quality of sound signals recorded on a cinema film is attached with special importance, the magnetic sound recording system is used hitherto. However, with recording sound signals magnetically on a cinema film, many other troublesome matters are met. The first of these matters is that excessive cost is required for coating a magnetic sound track on a cinema film. The second of these matters is that the base film on which a magnetic layer is disposed is fairly thicker than that of a magnetic tape used exclusively for recording sound signals, so that a magnetic head cannot be attached sufficiently to the magnetic layer deposited on the film transported continuously. Accordingly, in the magnetic sound recording system used for the cinema film, high pressure is applied between the magnetic head and the magnetic layer, so that magnetic powders plucked off from the magnetic layer are apt to close a gap of the magnetic head. The third of those matters is that a fairly heavy frictional load of a film transporting mechanism applied with the high pressure mentioned above is apt to cause uneven speed on transport of the film, so that wow and flutter are apt to occur. The fourth of those matters is that a lifetime is shortened owing to the high pressure mentioned above, so that the increase of running cost, especially for a cinema film of 70 mm width type, cannot be neglected. The last of those matters is that magnetic patterns recorded on the film are fairly unstable in comparison with the optical pattern recorded on the film, so that the sound signals recorded magnetically on the film are in danger of being vanished by a strong magnetic field applied externally.
On the contrary, the optical sound recording system according to the present invention does not at all meet the troublesome matters mentioned above concerning the magnetic sound recording system used for the cinema film, and is possessed of essential superiority, so that an extreme worth in practical use can be obtained by the present optical sound recording system in case the inferiority of quality as mentioned earlier can be removed.
On the other hand, it is taken into consideration hitherto for improving the quality mentioned above concerning the optical sound recording system of variable area type to apply a pulse-width modulation used for other technical fields, for instance, an electron beam recording. However, any practical configuration of equipments wherein a pulse-width modulation system being preferable for recording sound signals optically can be realized is not yet known at all.